1. Field of the Invention
The invention relates to a display system and, more particularly, to an image data control apparatus in a display system using a ferroelectric liquid crystal having a memory performance.
2. Related Background Art
In recent years, in a liquid crystal display system which is required for a personal computer (PC), a work station (WS), or the like, a large display screen and high resolution have been realized year by year, and compatibility with an existing PC or WS is also required.
When explaining a PC/AT machine made by IBM Corporation, which is generally widely used as an example, as display modes which are used in a display system, there are display modes of the adapter specification for an image such as CGA (Color Graphics Array), EGA (Enhanced Graphics Array), VGA (Video Graphic Array), 8514/A, and the like made by IBM Corporation. The resolutions and the numbers of colors which can be displayed in the above display modes are respectively different.
As a system which can display such various display modes by a single display system, for instance, there have been known "Multisync II", "Multisync 3D", "Multisync 4D", "Multisync 5D", and the like made by NEC Corporation as CRT (Cathode Ray Tube) display systems. In the liquid crystal display system which can realize a PC or WS of the lap-top type, there is a problem such that it is difficult to enable various kinds of display modes to be displayed by a single display system.
Particularly, in the display system using the ferroelectric liquid crystal, as compared with the case of the CRT display system, the following considerations are needed with respect to the display color and the resolution and the compatibility with the CRT display system is small.
(1) With respect to the display color:
In the ferroelectric liquid crystal display device, a bistable orientation state, that is, a binary display state is realized as disclosed in U.S. Patent Ser. No. 4,367,924 by Clerk et al. or the like. In other words, there is a problem such that it is difficult to realize a tone expression by a control of an applied voltage or the like. To realize the tone display by such a binarization display device, the tone display (area tone display) is realized by changing a ratio of the numbers of white and black dots.
On the other hand, in the ordinary display mode, the CRT display system has been designed so as to emit a light by a luminance corresponding to a tone for one dot (luminance tone).
In the case where various display modes of different display colors are displayed by the ferroelectric liquid crystal display system, therefore, the luminance tone cannot be used, so that the tone information needs to be changed to the data of the area tone display system for every ferroelectric liquid crystal display system in accordance with each display mode.
(2) With respect to the resolution:
When the display mode used in the CRT display system is applied to the ferroelectric liquid crystal display system of a certain resolution (e.g., 640.times.480 pixels), in the case of the display mode of a resolution (e.g., 320.times.200 pixels) lower than the above resolution, surplus pixels occur. Or, in the case of the display mode of a resolution (e.g., 720.times.400 pixels) higher than the above resolution, the pixels lack in the horizontal direction. In such cases, it is necessary to expand or compress the image data in the vertical or horizontal direction.
On the other hand, since the ferroelectric liquid crystal panel has a memory effect, there is a problem such that a display state before switching in a border portion which occurs upon switching of the display mode mentioned above remains as an after image without being erased.